子控制器内如何访问父控制器中的变量

<div ng-controller="OparentController as pCtrl">
    <p>直接访问父控制器的值：{{user}}</p>
    <div ng-controller="OchildController as cCtrl">
        <p>嵌套如何访问。。父控制器的值：失败：<input ng-model="pUser"/>成功：<input ng-model="obj2.pUser"/></p>
        <p>子控制器的值：{{user}}</p>
        <button ng-click="modiV()">clk:子控制器内修改父控制器内（作用域）的值</div>
    </div>
</div>
<script>
    var a10086=angular.module('a10086',[]);
    a10086.controller('OparentController',function($scope){
        $scope.user='parent_李四';
        $scope.msg={a:'parent_李四',b:4,c:[1,34]};
    }).controller('OchildController',function($scope){
        $scope.user='child_张三';
        $scope.pUser=$scope.msg.a;
        $scope.obj2={
            pUser:$scope.msg.a
        };
        $scope.modiV=function(){
            $scope.msg.a="parent_李四 is modify";
            $scope.obj2.pUser="parent_李四 is modify---obj";
            console.log('子控制没有msg，才顺着原型链修改了:',$scope.msg, $scope.pUser);
        }
    });
</script>